Open Loop vs. Closed Loop Controls

In a recent webinar regarding Process Documentation, I received this question regarding process controls…
During an In-Mold Rheology Test, should we use the closed loop or open loop option on our machines? What is the difference?
note: For those new to the industry, open loop process control uses a set pressure to force polymer into the mold during first stage injection – adversely, closed loop control uses a velocity setpoint to control the filling of the mold.
My Response
Obviously, if your molding machine is limited to open-loop control, the you can perform the test, but make a note regarding the process control type. Since the screw speed drops as the mold fills, the apparent shear rate you calculate is less accurate since it represents an average of the shear the polymer encounters.
A more realistic representation of apparent shear rate the polymer encounters results from closed loop process control. This is because the screw speed is controlled, representing a more realistic picture of the shear thinning behavior of the polymer. This also gives you much more control over the process over the long term since changes in material viscosity will cause virtually no change to the first stage fill time.
Additional Thoughts
There are some advancements in machine technology which have little to no benefit, these include: back pressure profiling, hold pressure profiling, and cushion control. On the other hand, closed loop process control for 1st stage mold filling is one of the most beneficial advances in injection molding technology over the past 50 years.
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